The present invention relates to stainless steel sinks which have improved scratch and stain resistance.
Consumers desire that their kitchen sink appear “like new” over the full lifetime of the sink. However, basin surfaces of such sinks are typically subjected to contact with sharp utensils and heavy pots, certain hard ceramic dishware, harsh chemicals and aggressive cleaning practices.
For example, consumers sometimes try to scrub sinks with highly abrasive steel wire scrubbing pads. Also, sharp carving knives are occasionally accidentally dropped into, or banged against top edges of, such basins. Further, ceramic coffee mugs and dinner plates have a tendency to scratch soft sink surfaces when accidentally dragged against them as the cups and dishes are being washed in the sink.
Conventional sink materials that are the most susceptible to scratching (and in some cases susceptible to staining) include plastics (e.g. thermoformed acrylics) and certain stainless steels. As a result, enameled cast iron is often more sought after in preference to sinks made of such other materials. However, there can be a considerable cost differential between enameled cast iron sinks and stainless steel or plastic sinks.
One approach to reduce the visual prominence of scratching on a conventional stainless steel sink involves pre-texturing the surface of the sink basin. See e.g. WO 2006064883. This idea of hiding the scratching “in plain sight” reduces the visibility of some scratches. However, it also limits the aesthetic design options.
Another alternative is to alter the chemical content of the stainless steel itself. With certain additives inside the steel the hardness of the steel can be adjusted somewhat. However, this approach has its limitations, and when pushed too far can adversely affect the production process or performance characteristics.
There have also been attempts to coat such sinks with a protective surface coating. In some cases this can adversely affect the appearance of the sink. In others, this can increase costs which are not sufficiently justified by the degree of improvement.
For example, Toto has proposed using microscopic beads of fluorinated polymer dispersed in a layer of ceramic silica as a protection for sinks. However, this coating is susceptible to scratching from certain sources.
As another example U.S. patent application publication 20050154112 described a coating based on sol-gel processing and contained silicon and zirconium as the primary film-forming constituents. Although silicon dioxide and zirconium dioxide are very hard materials, the surface of a stainless steel sink coated with this sol-gel material can still be too easily scratched/dented with a stainless steel knife when the layer is thin, and thicker coating layers can be susceptible to cracking.
Others have tried use of silicate coatings. See e.g. EP 1854908. If the silicate coating is too thin, a stainless steel knife will easily scratch the surface. Above a certain thickness, the coating will crack or discolor if processed in a normal manner.
Still others have tried depositing a titanium nitride coating on stainless steel sinks. This process is very expensive and time consuming.
In U.S. Pat. No. 1,511,555 there was described the deposition of metallic chromium on steel in an electroplating process to create a tool. However, there was no suggestion to apply these teachings to the sink art, much less a suggestion as to how to optimize coating thicknesses to accommodate wear variations at different locations along the sink.
U.S. Pat. No. 2,950,839 taught that stainless steel bread pans could have plated thereon a thick intermediate layer of chromium, followed by treatment of the chromium layer to form an oxide on the chrome layer's outer surface. This patent was focused on how to permit easy removal of food from the pan, and involved modified surface appearance.
U.S. Pat. No. 3,730,489 taught use of a chrome layer on a stainless steel vibrating board associated with a tank. The thickness of the layer of chrome was set for various reasons associated with the vibrating board's function. In any event the tank itself was not coated.
There have also been prior art suggestions to include substantial amounts of chromium in the chemical formula for stainless steel itself, and then use that chromium steel to form kitchen sinks. However, these sinks are the very ones in need of considerable improved scratch and stain resistance.
Also, there have been a variety of teachings and suggestions to apply chrome finishes to certain metal products to improve decorative finish (e.g. an auto bumper). However, particularly recently, some have suggested that use of chrome coatings should be minimized or avoided due to environmental difficulties experienced in handling one of the typical reagents used to form such coatings in some processes.
Accordingly, there exists a need for improving the resistance characteristics of stainless steel sinks, without adversely compromising other desired features and attributes of the sink or how it is made.